I don’t think I’d use the Source Engine. If only because the licensing details aren’t exactly crystal clear. It’s great that it works on OSX and Linux though.
It really depends on the game I was making. If I was making a 2D game I’d be tempted to make my own engine.. but I would probably use Unity.. just so I didn’t have to make my own level editors and whatever.
I don’t have much interest in making iOS/Android games. It is appealing in the sense that there’s a really huge market.. but there’s enough people chasing that crowd that you’re pretty much guaranteed to fail unless you spend hundreds of thousands on marketing, and bribing review sites to mention your game.